The success of radiotherapy, such as stereotactic body radiation therapy (SBRT), generally relies on the accuracy of the localization of the tumor and surrounding organs at risk (OAR). Often, this is based on a physician's judgment. Conventionally, a physician subjectively determines a treatment phase for which the treatment plan is based from the obtained 4D Computer-Tomography (CT) image data and associated respiratory gating signals from external surrogates. The radiation therapy is then delivered to the tumor only during the physician selected treatment phase. However, errors in selecting the treatment phase can have devastating consequences for the surrounding normal tissue, for example, by resulting in severe toxicity from the radiation treatment.